The present invention relates, in general, to a multilayer organic device and a method of fabricating such a device. More particularly, such a multilayer organic device can be an organic electroluminescent device comprising a polymeric light emitting diode (PLED) or a photoresponsive device such as a solar cell forming a polymeric organic photovoltaic (OPV) device.
Semiconducting polymers make remarkably effective substitutes for conventional inorganic semiconductors in a range of optoelectronic devices. Such devices can include light emitting diodes (LEDs), photovoltaic (PV) diodes, field effect transistors (FETs), and lasers. Conjugated polymers offer considerable material advantages over inorganic semiconductors including chemically tunable optoelectronic properties and low-temperature, solution-based processing suitable for printed electronics.
A single layer organic device comprises a pair of electrodes and an active material generally copolymerized or blended to achieve a desired emission wavelength and to balance injected charge carriers. However, different electron and hole mobility properties of the active material, non-ideal electron and hole injection from the electrodes and quenching by electrodes make single layer organic devices inefficient for many applications.
In contrast to such single layer organic devices, so-called multilayer organic devices comprise at least two layers of an organic material between the electrodes. These multilayer organic devices can be made more efficient than single layer organic devices, because one can engineer the outer layers next to the electrodes to enhance either hole or electron injection, which in turn reduces the turn on voltage in the case of emissive devices or facilitate carrier collections in the case of organic solar cells. One limitation on manufacturing processes involving semiconducting polymers is the difficulties in preparing multilayer organic devices. A challenge of forming such multilayer organic devices is one of building layers of the device without intermixing of layers or damaging existing layers during a layer depositing process.
It is well known in the state of the art to form efficient multilayer organic devices by sublimation of organic molecules in a vacuum and the subsequent deposition into different layers. A drawback of such vacuum deposition technique is that it requires both the use of expensive machinery and in addition leads to a high wastage rate of material. Most importantly, fabricating multilayer organic devices via vacuum deposition is fairly feasible for organic semiconductor material with comparatively larger molecular weight, such as oligomers or polymers since they cannot be thermally evaporated without destruction of the organic material. Thus, organic semiconductor material with comparatively larger molecular weight are preferentially processed in solution, i.e. via wet-chemical deposition techniques including spin coating, ink jet, screen or roll-to-roll printing.
From a cost point of view, solution processing techniques such as wet-chemical deposition are the more attractive technique for mass production, especially for the production of organic devices of larger size. A single layer of semiconducting polymers can be laid down in solution comparatively cheaply in a well controlled way, without the requirement of costly and complex machinery. However, the fabrication of multi-layer structures from solution faces severe challenges. In depositing sequential layers, there is the problem that the solvents might re-dissolve previously deposited layers, resulting potentially in intermixing of layers or damaging of existing layers during a layer depositing process. It might also cause process irreproducibility or reduced efficiency of the resulting devices. In order to avoid these drawbacks, it is therefore important to ensure that layers already deposited from solution are resistant to the solvents used to deposit subsequent layers.
As a matter of fact, only few solution processed multilayer devices have been reported due to the nature of their fabrication. One way to get around the problem of re-dissolution of previously deposited layers is to use materials that can be deposited from orthogonal solvents i.e. solvents that differ distinctly regarding their solubility properties, e.g. by their polarities. An example thereof are polar and non-polar solvents. In general, polar materials can be dissolved in polar solvents while non-polar materials can be dissolved in non-polar solvents. Polar solvents, however, typically do not dissolve non-polar materials and, vice versa, non-polar solvents do not dissolve polar materials.
In principle, a multilayer device could be created by applying layers of polar and non-polar materials in alternating layers. Thereby, polar materials are dissolved in a polar solvent and non-polar materials are dissolved in a non-polar solvent and these two materials are spin coat in consecutive steps upon each other.
The options for the manufacturing of multilayer organic devices regarding orthogonal solvents are quite limited. There is a big disadvantage with this approach. Typically most conjugated, semiconducting polymers are preferentially soluble in organic and/or non-polar media. Thus, in order to create multilayer organic devices with at least two layers of semiconducting polymers, one of the polymers needs to be chemically modified in order to be soluble in a polar solvent such as water. In general, changing the polarity of the semiconducting polymer is not an easy task and will usually be accompanied by a deterioration of the material. Thus, the efficiency and/or the operating lifetime of the device might be negatively affected.
An example where this strategy of using orthogonal solvents is realized for a polymeric light emitting diode is given in Gong et al, ‘Multilayer Polymer Light-Emitting Diodes: White-Light Emission with High Efficiency’, Adv. Mater. 2005, 17, 2053. There, a three-layer device was fabricated consisting of an emissive layer of a polyfluorene host and an iridium dopant (both soluble in organic solvents; insoluble in water) sandwiched between a water-soluble PVK derivative as a hole-transport layer (HTL) and a water-soluble PBD derivative as an electron-transport layer (ETL).
A different approach creating multilayer organic devices involves rendering deposited layers insoluble through post-deposition treatment. An example thereof for a fully solution-processed light emitting device is disclosed in prior published European patent application EP 1 753 047 A1. Hereby, the general idea is to apply a crosslinkable material for the individual layer and initiate crosslinking after the material is deposited by thermal, chemical or other irradiative means such as ultra-violet curing. By this each deposited layer becomes insoluble to the solvents used for depositing the subsequent layer.
FIG. 15 shows a cross-sectional view of an electroluminescent device according to the above mentioned prior art which is disclosed in prior published European patent application EP 1 753 047 A1. The electroluminescent device may represent one pixel or sub-pixel of a larger display or part of lighting source. The device 405 includes a first electrode 411 on a substrate 408. One or more organic materials are deposited on the first electrode to form one or more organic layers of an organic stack 416. The organic stack 416 includes in particular a hole injection layer (HIL) 417 and an emissive layer (EML) 420. The OLED device 405 also includes a second electrode 423 on the top of the organic stack 416. Other layers than that shown in FIG. 15 may also be added including barrier, charge transport/injection, and interface layers between or among any of the existing layers as desired. According to the teaching of EP 1 753 047 A1, the hole injection layer is fabricated by using a crosslinkable hole injection/transport material doped with conductivity dopants.
As mentioned above, crosslinking is affected by thermal, chemical or other irradiative means such as ultra-violet curing so that each deposited layer becomes insoluble to the solvents used in a layer deposited upon it such as an emissive layer or subsequently deposited electron transport layer. This may include the use of ultraviolet curable inks, crosslinkable side chains, crosslinkable chain end groups or monomers which can be cross-linked into polymers for example.
However, the presence of crosslinkable material or the use of processes such as ultra-violet curing can damage and/or reduce the efficiency of the organic semiconductor layers. In addition, the operating lifetime of the multilayer organic device may be adversely affected.
In summary, in the past years different approaches have been proposed to create multilayer organic devices using solution-based processes. However, the known approaches suffer from a plenty of disadvantages which have been mentioned above. Despite continued research efforts over years, no fully convincing strategy has been found which allows the manufacture of efficient, solution processed multilayer organic devices.